Nearest star's wobbles could reveal Earth's twin

Another Earth may be orbiting the star next door, and we could detect its presence within a few years, a new study argues. A telescope trained permanently on Alpha Centauri should be able to pick up the slight stellar wobbles induced by a small, rocky, Earth-like planet.

Alpha Centauri lies just over 4 light years away and is the closest star system to the Sun. It appears to be a triple system, with two Sun-like stars orbiting each other relatively closely (about 23 times the Earth-Sun distance). The two stars have high concentrations of heavy elements, which is characteristic of stars that are born surrounded by dusty, planet-forming discs.

Previous computer simulations suggested terrestrial planets probably formed around one or both stars. That is borne out by the work of Javiera Guedes at the University of California, Santa Cruz (UCSC), US, and colleagues, who have gone a step further and worked out how to detect such planets.

"If our understanding of terrestrial planet formation is at all correct, then there should definitely be terrestrial planets orbiting both members of the Alpha Centauri binary pair," team member Greg Laughlin of UCSC told New Scientist.

What's more, any such planets might boast the conditions thought to be necessary to support life. In the team's simulations of planet formation around the smaller star, Alpha Centauri B, an Earth-like world often coalesced in or near the star's habitable zone, where liquid water could exist on the planet's surface.

Finding these planets could be time-consuming, but it does not require any new techniques, they say. They suggest using the "radial velocity" method, which looks for spectral signs that a star is wobbling due to gravitational tugs from an orbiting planet.

Calm atmosphere

The method has discovered most of the 228 known exoplanets. But until now, it has turned up only giant Jupiter-like planets, which produce relatively large wobbles in their host stars.

"Our aim is to find rocky planets by muscling up the same technique that has been so successful in finding more massive planets," says team member Debra Fischer of San Francisco State University in California, US.

Laughlin realised that Alpha Centauri B was an exceptionally good target for this method, in part because it is a calm star. The atmospheres of most stars of its type churn more violently, which would obscure the slight movement caused by orbiting Earth-like planets.

And because it is so near to Earth, Alpha Centauri B is very bright. That means astronomers can rapidly capture a precise spectrum of its light, which is ideal for measuring small Doppler shifts due to terrestrial planets.

Faint signal

Even so, the researchers think they will need several years of data to smooth out random noise in their observations to be able to spot the faint signal of another Earth. That's because a terrestrial planet would cause Alpha Centauri B to wobble at speeds of only about 10 centimetres per second.

Laughlin and his team will start to monitor Alpha Centauri in May, using a 1.5-metre telescope at the Cerro Tololo Inter-American Observatory in Chile. As well as searching for planets, their observations will be used to analyse the stars' natural oscillations, which could reveal details about their internal structures.

Not all astronomers are convinced by the simulations that Alpha Centauri should host terrestrial planets. "I tend to be sceptical of planet-formation models," says Sara Seager of MIT in Cambridge, US, who did not take part in the study.

But Seager is impressed with the second part of the paper, demonstrating that these planets should be detectable. "It is tremendously exciting that we can search for an Earth cousin in a habitable zone of a nearby star with current technology," she told New Scientist.

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Though it looks like a single star, Alpha Centauri (the bright spot left of centre) is actually a triplet (Image: Claus Madsen/ESO)